1. Field of the Invention
A monocoque glass clinical thermometer in which the bulb and stem are formed from the same piece of glass and in which the exterior of the bulb is of circular, preferably cylindrical, section and is completely free of protuberances that give rise to irritation in normal use of the thermometer.
2. Description of the Prior Art
In the prior art it has been conventional to form clinical glass thermometers by fusing to the bottom of the stem a previously formed bulb of thin section in suitable configuration. The stem was made from standard Corning lead thermometer glass and the bulb was made from a different type of material which was dimensionally stable upon stress relief at room temperature. Clinical thermometers so made had a high reject rate which was caused from internal undercuts that arose from misalignment of the walls of the stem and the bulb. These undercuts resulted in air being trapped therein; the air caused splits or gaps in the mercury column.
Furthermore, particles of dust often would become lodged in the thermometer during the fusing of the bulb to the stem, particularly so because the bulb was of comparatively large diameter and air-borne particles of dust easily could float into the same. These particles tended to enter the capillary bore during loading of the thermometer and also use of the thermometer, thereby creating thermometers known as "hard shakers", these being thermometers in which the mercury column would not readily recede after using the same to measure a temperature even when they were whipped quite rapidly.
To overcome the difficulty with thermometers formed from separate stem blanks and bulbs of different materials, it has been proposed to form thermometers as shown in McGill U.S. Pat. No. 3,285,727. In this patent the bulb of the finished thermometer was in one piece with the glass of the stem. Indeed, the bulb was formed from one end of a thermometer blank. Pursuant to the patent, the thermometer blank was held upright and the top portion heated to flow temperature of the glass. The upper end of the blank was cut with a gas jet which concurrently closed the capillary bore. Pressurized gas was introduced into the bore resulting in expansion of the closed end of the bore and the corresponding portion of the side walls of the blank. Next, the bulb was rotated between cylindrical rollers to bring the bulb to a desired shape. Alternatively, the bulb was rotated with non-cylindrical dies to provide bulbs of different shapes. In this method, however, the rotation of the stem and of the rotary dies caused the opaque strip against which the mercury column was to be read to be severely twisted resulting in an unsightly spiral within the bulb. Furthermore, the use of a rotating blank and a rotating die member or members resulted in unevenness of the walls of the thermometer bulbs which made them more susceptible to breakage. Still further, this method did not provide a desired smooth shape for the extreme tip of the bulb. In addition, the apparatus to form the aforesaid steps was relatively expensive and the process for operating the apparatus was expensive and required the use of skilled help, thus increasing the price of the thermometer. Finally, the rejects of thermometers so made were quite high, which, too, increased the expense of such thermometers as sold to the public.
It also has been proposed in a Chancy U.S. Pat. No. 2,990,648 to form bulbs on thermometer stems which bulbs were made of the same glass as the stems and, indeed, in one piece therewith. These thermometers used as gas flame to seal the lower end of a vertically held thermometer blank, slightly expanded the bore at the closed end by introducing gas under pressure into the bore, and then while the bulb still was at a temperature sufficiently high to enable the glass therein to flow, introduced the bulb into a split cavity mold where the bulb was inflated to the configuration of the inside of the mold. The method disclosed by Chaney was used only in connection with non-clinical thermometers. The stem and the bulb had a flat side designed to be placed against a reading scale. The stem never was marked, as was conventional with a clinical thermometer, either with recessed markings or stain markings. The method disclosed by Chaney had the serious defect that the cavity left flash marks on the bulb which, of course, was no particular problem insofar as Chaney was concerned because a commercial thermometer never is intended to be inserted into a body orifice, so that the flash on the bulb never created an irritation as it would have if it were on a bulb in a clinical thermometer.